The present invention relates to an energy cable. In particular, the present invention relates to a cable for transporting or distributing electric energy, especially medium or high voltage electric energy, said cable having a sheath which guarantees to the cable improved flexibility combined with high mechanical strength and thermopressure resistance. Said cable may be used for either direct current (DC) or alternating current (AC) transmission or distribution.
Cables for transporting electric energy generally include at least one cable core. The cable core is usually formed by at least one conductor sequentially covered by an inner polymeric layer having semiconductive properties, an intermediate polymeric layer having electrically insulating properties, an outer polymeric layer having semiconductive properties. Cables for transporting medium or high voltage electric energy generally include at least one cable core surrounded by at least one screen layer, typically made of metal or of metal and polymeric material. The screen layer can be made in form of wires (braids), a tape helically wound around the cable core or a sheet longitudinally surrounding the cable core. The polymeric layers surrounding the at least one conductor are commonly made from a polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked, as disclosed, e.g., in WO 98/52197. The crosslinking step, carried out after extruding the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during continuous use and with current overload. To address requirements for materials which should not be harmful to the environment both during production and during use, and which should be recyclable at the end of the cable life, energy cables have been recently developed having a cable core made from thermoplastic materials, i.e. polymeric materials which are not crosslinked and thus can be recycled at the end of the cable life.
In this respect, WO 02/03398 and WO 02/27731, both in the Applicant's name, disclose cables comprising at least one electrical conductor and at least one extruded covering layer based on thermoplastic polymer material in admixture with a dielectric liquid, wherein said thermoplastic material comprises a propylene homopolymer or a copolymer of propylene with at least one olefin comonomer selected from ethylene and an alpha-olefin other than propylene, said homopolymer or copolymer having a melting point greater than or equal to 140° C. and a melting enthalpy of from 30 J/g to 100 J/g. Alternatively, as thermoplastic base material, a propylene homopolymer or copolymer as hereinbefore defined can be used in mechanical mixture with a low crystallinity polymer, generally with a melting enthalpy of less than 30 J/g, which mainly acts to increase flexibility of the material. The quantity of low crystallinity polymer is generally less than 70 wt %, and preferably from 20 to 60 wt %, on the total weight of the thermoplastic material.
WO 04/066318, in the Applicant's name, refers to a cable comprising at least one electrical conductor and at least one extruded covering layer based on a thermoplastic polymer material in admixture with a dielectric liquid, wherein said thermoplastic polymer material is selected from:                (a) at least one propylene homopolymer or at least one copolymer of propylene with at least one olefin comonomer selected from ethylene and an alpha-olefin other than propylene, said homopolymer or copolymer having a melting point greater than or equal to 130° C. and a melting enthalpy of from 20 J/g to 100 J/g;        (b) a mechanical mixture comprising at least one propylene homopolymer or copolymer (a) and (c) at least one elastomeric copolymer of ethylene with at least one aliphatic alpha-olefin, and optionally a polyene. The elastomeric copolymer of ethylene (c) has a melting enthalpy of less than 30 J/g. The quantity of said elastomeric copolymer (c) is generally less than 70% by weight, preferably of from 20% by weight to 60% by weight, with respect to the total weight of the thermoplastic base material.        
To mechanically protect the cable core against impacts and compressions, generally energy cables further comprise at least one polymeric coating layer placed in a radially outer position with respect to the cable core, and with respect to the screen layer when it is present, usually known as sheath or outer sheath.
U.S. Pat. No. 4,348,459 relates to a thermoplastic elastomeric composition suitable for making electrical plugs, connectors and cable jacketing comprising a blend of:                (a) ethylene-propylene-non-conjugated diene terpolymer rubber having an ethylene content of from 70 to 85% by weight and a gel content of from 10 to 45%, preferably 15 to 35%, by weight measured in cyclohexane at room temperature;        (b) an essentially crystalline propylene polymer;        (c) a naphthenic or paraffinic extender oil; and        (d) an inorganic (non-black) filler suitable to absorb at least a portion of said extender oil (c);wherein the weight ratio of (a)/(b) is from 90/10 to 25/75, preferably from 80/20 to 40/60, the concentration of (c) is from 20 to 100 parts per 100 parts by weight of (a) plus (b), the concentration of (d) is from 10 to 100 parts per 100 parts by weight of (a) plus (b), and the weight ratio of (c)/(d) does not exceed 2.5/1.        
The above compositions are characterized by unusual resistance to high temperatures and they therefore provide an improved margin of safety over material commonly used for electrical insulation, especially polyvinylchloride.
WO 97/03124 relates to a cable-sheathing composition, as well as the use thereof as outer sheathing for a power cable or a communication cable. In particular, this document discloses a cable-sheathing composition consisting of a multimodal olefin polymer mixture having a density of about 0.915-0.955 kg/dm3 and a melt flow rate of about 0.1-0.3 g/10 min, said olefin polymer mixture comprising at least a first and a second olefin polymer, of which the first has a density and a melt flow rate selected from (a) about 0.930-0.975 kg/dm3 and about 50-2000 g/10 min and (b) about 0.88-0.93 kg/dm3 and about 0.1-0.8 g/10 min.
The above multimodal olefin polymer mixtures would result in improved cable-sheathing compositions, especially as regards shrinkage, environmental stress cracking resistance (ESCR) and processability.
U.S. Pat. No. 5,718,974 relates to a cable having a jacket comprising an in situ blend of two co-polymers of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms, said blend having a Mw/Mn ratio in the range of about 8 to about 22; a melt index in the range of about 0.2 to about 3.5 grams per 10 minutes; a melt flow ratio in the range of about 55 to about 135; a molecular weight in the range of about 90,000 to about 250,000; and a density of at least 0.915 kg/dm3.
The above blend would be endowed with high tensile strength, high elongation, and improved low temperature brittleness as compared with linear low density polyethylene (LLDPE) while maintaining the other advantageous properties of LLDPE, and which is based on a more easily extrudable resin comparable to LLDPE.
Additives, which can be introduced into the in situ blend, are exemplified, inter alia, by plasticizers. Additives can be added in amounts ranging from about 0.01 to about 10 parts by weight for each 100 parts by weight of the base resin (from 0.01% and 9.1% by weight). Said amount is referred to all of the additives without any information about the quantity of plasticizers to be used.
US 2006/0189744 relates to articles comprising plasticized thermoplastic polyolefin compositions comprising one or more thermoplastic polyolefins, one or more non-functionalized plasticizers (NFP), and one or more nucleating agents. The thermoplastic polyolefin may be a polypropylene or propylene polymer, i.e. a polymer made of at least 50 mole % of propylene units and having less than 35 mole % of ethylene units. Alternatively, the polyolefin may be a polyethylene or ethylene polymer, i.e. a polymer made of at least 50 mole wt % of ethylene units and having less than 20 mole % of propylene units.
The NFP is a hydrocarbon liquid which does not include to an appreciable extent functional groups selected from hydroxide, aryls and substituted aryls, halogens, alkoxys, carboxylates, esters, carbon unsaturation, acrylates, oxygen, nitrogen, and carboxyl. Preferably, the NFPs include isoparaffins, PAOs, Group III basestocks or mineral oils, high purity hydrocarbon fluids derived from a so-called Gas-To-Liquids processes, and mineral oils with a viscosity index greater than 100, pour point less than −20° C., specific gravity less than 0.86, and flash point greater than 200° C. The amount of NFPs may vary within a wide range of values, generally from 60 to 0.1 wt %, or even at 5 wt % or less, based upon the total weight of the composition.
The articles that can be produced by means of the above compositions are, among many others, wire and cable jacketing.
The teachings of the above-mentioned document are directed to solve a problem regarding satisfactory aesthetics in a molded article, because plasticization causes certain types of thermoplastic polyolefins, especially polypropylene, to exhibit undesirable optical and/or tactile properties, especially at low temperature.